1. Field of the Invention
The present invention relates to drill bits used to create boreholes through a material and, more particularly, to such drill bits that are used in the exploration for subterranean hydrocarbon deposits.
2. Setting of the Invention
In the exploration for subterranean hydrocarbon deposits, a rotating drill bit is used to create a borehole through the earth's subsurface formations. The users of such drill bits and drill bit manufacturers have found that by more precisely controlling the weight-on-bit (WOB) and increasing the rotational speed (RPM) that highly desirable increased drill bit penetration rates can be achieved. However, as the RPM is increased the effective life of the drill bits has decreased dramatically because the cutting elements on the drill bits become very quickly cracked and can be violently torn from the drill bits.
Numerous studies have been made to find out what causes such destruction to the cutting elements. The inventors hereof have previously found that a substantial portion of the destructive forces are generated by radial imbalance forces that cause a drill bit to rotate about a rotational axis offset from the geometric center of the drill bit in such a way that the drill bit tends to wobble or "backwards whirl" about the borehole. This backwards whirling causes the center of rotation to change dramatically as the drill bit rotates about the borehole. Thus, the cutting elements travel faster, sideways, and backwards and thus are subject to greatly increased impact loads which cause the destruction of the cutting elements.
More specifically, circumferential drilling imbalance forces exist to some degree on every drill bit and these forces tend to push the drill bit towards the side of the borehole. In a typical drill bit, gauge cutting elements are designed to cut the edge of the borehole. During the cutting process, the effective friction between the cutting elements near the gauge area increase and, thus, the instantaneous center of rotation becomes some point other than the geometric center of the drill bit. When this happens, the usual result is for the drill bit to begin to backwards whirl around the borehole. This whirling process regenerates itself because sufficient friction is always generated between the drill bit gauge area and the borehole wall, no matter what the orientation of the drill bit, from the centrifugal forces generated by the rapid acceleration of the drill bit.
Various methods and equipment have been proposed to eliminate or reduce these imbalance forces, including using dynamically balanced lower drillstring assemblies and very precisely aligning the cutting elements to reduce imbalance forces.
Various designs of drill bits have been developed to improve penetration rates by aligning the cutting elements in a plurality of equal radius sets, with each set being in overlapping radial relationship. One such drill bit design is disclosed in U.S. Pat. No. 4,545,441. Further, various attempts at improving cutting element life have been made by varying the back or side rake or angle of attack of the cutting elements, i.e., the angle at which the face of the cutting element addresses the formation with respect to the formation surface. The benefits of varying such back rake angles are disclosed in "The Effect Of Back Rake Angle On The Performance Of Small-Diameter Polycrystalline Diamond Rock Bits: ANOVA Tests," Journal of Energy Resources Technology, Vol. 108, No. 4, pp. 305-309, December 1986; U.S. Pat. No. 4,660,659; U.S. Pat. No. 4,440,247; U.S. Pat. No. 4,186,628 and U.S.S.R. Pat. No. 395,559. The effects of varying side rake angles is disclosed in Hunnj SPE-10152 (1981).
There is no disclosure or suggestion in any of the above-identified article or patents of arranging cutting elements specifically to prevent or reduce the effects of destructive bit whirl. There is a need for a drill bit design which incorporates features designed specifically for preventing bit whirl and improving cutting element life.